Lignin-coated cellulose fibers from lignocellulosic biomass

ABSTRACT

A process is provided for producing a lignin-coated cellulose material, comprising: pre-extracting a lignocellulosic biomass feedstock in the presence of steam or hot water, depositing lignin from the liquid onto a surface of solids to generate a lignin-coated intermediate material; optionally drying the intermediate material; digesting the lignin-coated intermediate material in the presence of an acid, a solvent for lignin, and water, wherein the rate of delignification of surface lignin is lower than the rate of delignification of bulk lignin; and recovering a hydrophobic lignin-coated cellulose material. In some variations, part of the overall process is a combination of Green Power+® and AVAP® technologies. A cellulose-rich composition is provided, containing about 5 wt % to about 15 wt % total lignin, with particles having a higher average surface concentration of lignin compared to an average bulk (internal) concentration of lignin.

PRIORITY DATA

This patent application is a non-provisional application with priority to U.S. Provisional Patent App. No. 61/941,215, filed Feb. 18, 2014, which is hereby incorporated by reference herein.

FIELD

The present invention generally relates to fractionation processes for converting biomass into fermentable sugars, cellulose, and lignin.

BACKGROUND

Biomass refining (or biorefining) is becoming more prevalent in industry. Cellulose fibers and sugars, hemicellulose sugars, lignin, syngas, and derivatives of these intermediates are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint.

Lignocellulosic biomass is the most abundant renewable material on the planet and has long been recognized as a potential feedstock for producing chemicals, fuels, and materials. Lignocellulosic biomass normally comprises primarily cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are natural polymers of sugars, and lignin is an aromatic/aliphatic hydrocarbon polymer reinforcing the entire biomass network. Some forms of biomass (e.g., recycled materials) do not contain hemicellulose.

There are many reasons why it would be beneficial to process biomass in a way that effectively separates the major fractions (cellulose, hemicellulose, and lignin) from each other. Cellulose from biomass can be used in industrial cellulose applications directly, such as to make paper or other pulp-derived products. The cellulose can also be subjected to further processing to either modify the cellulose in some way or convert it into glucose. Hemicellulose sugars can be fermented to a variety of products, such as ethanol, or converted to other chemicals. Lignin from biomass has value as a solid fuel and also as an energy feedstock to produce liquid fuels, synthesis gas, or hydrogen; and as an intermediate to make a variety of polymeric compounds. Additionally, minor components such as proteins or rare sugars can be extracted and purified for specialty applications.

In light of this objective, a major shortcoming of previous process technologies is that one or two of the major components can be economically recovered in high yields, but not all three. Either the third component is sacrificially degraded in an effort to produce the other two components, or incomplete fractionation is accomplished. An important example is traditional biomass pulping (to produce paper and related goods). Cellulose is recovered in high yields, but lignin is primarily consumed by oxidation and hemicellulose sugars are mostly degraded. Approximately half of the starting biomass is essentially wasted in this manufacturing process. State-of-the-art biomass-pretreatment approaches typically can produce high yields of hemicellulose sugars but suffer from moderate cellulose and lignin yields.

There are several possible pathways to convert biomass into intermediates. One thermochemical pathway converts the feedstock into syngas (CO and H₂) through gasification or partial oxidation. Another thermochemical pathway converts biomass into liquid bio-oils through pyrolysis and separation. These are both high-temperature processes that intentionally destroy sugars in biomass.

Sugars (e.g., glucose and xylose) are desirable platform molecules because they can be fermented to a wide variety of fuels and chemicals, used to grow organisms or produce enzymes, converted catalytically to chemicals, or recovered and sold to the market. To recover sugars from biomass, the cellulose and/or the hemicellulose in the biomass must be hydrolyzed into sugars. This is a difficult task because lignin and hemicelluloses are bound to each other by covalent bonds, and the three components are arranged inside the fiber wall in a complex manner. This recalcitrance explains the natural resistance of woody biomass to decomposition, and explains the difficulty to convert biomass to sugars at high yields.

Fractionation of biomass into its principle components (cellulose, hemicellulose, and lignin) has several advantages. Fractionation of lignocellulosics leads to release of cellulosic fibers and opens the cell wall structure by dissolution of lignin and hemicellulose between the cellulose microfibrils. The fibers become more accessible for hydrolysis by enzymes. When the sugars in lignocellulosics are used as feedstock for fermentation, the process to open up the cell wall structure is often called “pretreatment.” Pretreatment can significantly impact the production cost of lignocellulosic ethanol.

One of the most challenging technical obstacles for cellulose has been its recalcitrance towards hydrolysis for glucose production. Because of the high quantity of enzymes typically required, the enzyme cost can be a tremendous burden on the overall cost to turn cellulose into glucose for fermentation. Cellulose can be made to be reactive by subjecting biomass to severe chemistry, but that would jeopardize not only its integrity for other potential uses but also the yields of hemicellulose and lignin.

Many types of pretreatment have been studied. A common chemical pretreatment process employs a dilute acid, usually sulfuric acid, to hydrolyze and extract hemicellulose sugars and some lignin. A common physical pretreatment process employs steam explosion to mechanically disrupt the cellulose fibers and promote some separation of hemicellulose and lignin. Combinations of chemical and physical pretreatments are possible, such as acid pretreatment coupled with mechanical refining. It is difficult to avoid degradation of sugars. In some cases, severe pretreatments (i.e., high temperature and/or low pH) intentionally dehydrate sugars to furfural, levulinic acid, and related chemicals. Also, in common acidic pretreatment approaches, lignin handling is very problematic because acid-condensed lignin precipitates and forms deposits on surfaces throughout the process.

One type of pretreatment that can overcome many of these disadvantages is called “organosolv” pretreatment. Organosolv refers to the presence of an organic solvent for lignin, which allows the lignin to remain soluble for better lignin handling. Traditionally, organosolv pretreatment or pulping has employed ethanol-water solutions to extract most of the lignin but leave much of the hemicellulose attached to the cellulose. For some market pulps, it is acceptable or desirable to have high hemicellulose content in the pulp. When high sugar yields are desired, however, there is a problem. Traditional ethanol/water pulping cannot give high yields of hemicellulose sugars because the timescale for sufficient hydrolysis of hemicellulose to monomers causes soluble-lignin polymerization and then precipitation back onto cellulose, which negatively impacts both pulp quality as well as cellulose enzymatic digestibility.

An acid catalyst can be introduced into organosolv pretreatment to attempt to hydrolyze hemicellulose into monomers while still obtaining the solvent benefit. Conventional organosolv wisdom dictates that high delignification can be achieved, but that a substantial fraction of hemicellulose must be left in the solids because any catalyst added to hydrolyze the hemicellulose will necessarily degrade the sugars (e.g., to furfural) during extraction of residual lignin.

Contrary to the conventional wisdom, it has been found that fractionation with a solution of ethanol (or another solvent for lignin), water, and sulfur dioxide (SO₂) can simultaneously achieve several important objectives. The fractionation can be achieved at modest temperatures (e.g., 120-160° C.). The SO₂ can be easily recovered and reused. This process is able to effectively fractionation many biomass species, including softwoods, hardwoods, agricultural residues, and waste biomass. The SO₂ hydrolyzes the hemicelluloses and reduces or eliminates troublesome lignin-based precipitates. The presence of ethanol leads to rapid impregnation of the biomass, so that neither a separate impregnation stage nor size reduction smaller than wood chips are needed, thereby avoiding electricity-consuming sizing operations. The dissolved hemicelluloses are neither dehydrated nor oxidized (Iakovlev, “SO₂-ethanol-water fractionation of lignocellulosics,” Ph.D. Thesis, Aalto Univ., Espoo, Finland, 2011). Cellulose is fully retained in the solid phase and can subsequently be hydrolyzed to glucose. The mixture of hemicellulose monomer sugars and cellulose-derived glucose may be used for production of biofuels and chemicals.

Commercial sulfite pulping has been practiced since 1874. The focus of sulfite pulping is the preservation of cellulose. In an effort to do that, industrial variants of sulfite pulping take 6-10 hours to dissolve hemicelluloses and lignin, producing a low yield of fermentable sugars. Stronger acidic cooking conditions that hydrolyze the hemicellulose to produce a high yield of fermentable sugars also hydrolyze the cellulose, and therefore the cellulose is not preserved.

The dominant pulping process today is the Kraft process. Kraft pulping does not fractionate lignocellulosic material into its primary components. Instead, hemicellulose is degraded in a strong solution of sodium hydroxide with or without sodium sulfide. The cellulose pulp produced by the Kraft process is high quality, essentially at the expense of both hemicellulose and lignin.

Sulfite pulping produces spent cooking liquor termed sulfite liquor. Fermentation of sulfite liquor to hemicellulosic ethanol has been practiced primarily to reduce the environmental impact of the discharges from sulfite mills since 1909. However, ethanol yields do not exceed one-third of the original hemicellulose component. Ethanol yield is low due to the incomplete hydrolysis of the hemicelluloses to fermentable sugars and further compounded by sulfite pulping side products, such as furfural, methanol, acetic acid, and others fermentation inhibitors.

Solvent cooking chemicals have been attempted as an alternative to Kraft or sulfite pulping. The original solvent process is described in U.S. Pat. No. 1,856,567 by Kleinert et al. Groombridge et al. in U.S. Pat. No. 2,060,068 showed that an aqueous solvent with sulfur dioxide is a potent delignifying system to produce cellulose from lignocellulosic material. Three demonstration facilities for ethanol-water (Alcell), alkaline sulfite with anthraquinone and methanol (ASAM), and ethanol-water-sodium hydroxide (Organocell) were operated briefly in the 1990s.

In view of the state of the art, what is desired is to efficiently fractionate any lignocellulosic-based biomass (including, in particular, softwoods) into its primary components so that each can be used in potentially distinct processes. While not all commercial products require pure forms of cellulose, hemicellulose, or lignin, a platform biorefinery technology that enables processing flexibility in downstream optimization of product mix, is particularly desirable. An especially flexible fractionation technique would not only separate most of the hemicellulose and lignin from the cellulose, but also render the cellulose highly reactive to cellulase enzymes for the manufacture of fermentable glucose.

Cellulose or cellulose derivatives can be used in a wide variety of applications such as polymer reinforcement, anti-microbial films, biodegradable food packaging, printing papers, pigments and inks, paper and board packaging, barrier films, adhesives, biocomposites, wound healing, pharmaceuticals and drug delivery, textiles, water-soluble polymers, construction materials, recyclable interior and structural components for the transportation industry, rheology modifiers, low-calorie food additives, cosmetics thickeners, pharmaceutical tablet binders, bioactive paper, pickering stabilizers for emulsion and particle stabilized foams, paint formulations, films for optical switching, and detergents.

For some cellulose applications, is would be beneficial to increase the hydrophobicity of the cellulose. Therefore, improved processes are needed in the art.

SUMMARY

The present invention addresses the aforementioned needs in the art.

In some variations, the invention provides a process for producing a lignin-coated cellulose material, the process comprising:

(a) providing a lignocellulosic biomass feedstock;

(b) pre-extracting the feedstock in the presence of steam or hot water, thereby generating a first solids stream and a first liquid stream, wherein the first liquid stream contains hemicelluloses and lignin;

(c) depositing at least some of the lignin, from the first liquid stream, onto a surface of the first solids stream to generate a lignin-coated intermediate material comprising cellulose-rich particles with a lignin coating;

(d) optionally drying the lignin-coated intermediate material;

(e) digesting the lignin-coated intermediate material in the presence of an acid, a solvent for lignin, and water, to generate a second solids stream and a second liquid stream, wherein during the digesting, the rate of delignification of surface lignin deposited from step (c) is lower than the rate of delignification of bulk lignin, thereby retaining at least a portion of the lignin coating; and

(f) recovering the second solids stream as a lignin-coated cellulose material, wherein the lignin-coated cellulose material is at least partially hydrophobic.

In some embodiments, step (b) further includes introducing an acid catalyst to enhance lignin deposition. Such an acid catalyst may be selected from the group consisting of acetic acid, formic acid, uronic acids, levulinic acid, sulfur dioxide, sulfurous acid, sulfuric acid, lignosulfonic acid, carbon dioxide, carbonic acid, and combinations thereof.

In some embodiments, some amount of drying is conducted in step (d). In these or alternative embodiments, water is removed by membranes, molecular sieves, centrifuges, or other means.

In some embodiments, the acid in step (e) is selected from the group consisting of sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, and combinations thereof. The acid is sulfur dioxide, in particular embodiments.

The process may further comprise hydrolyzing the hemicelluloses (in the first liquid stream) to produce monomeric sugars. The second liquid stream also typically contains hemicellulose oligomers; the process may further comprise hydrolyzing the hemicellulose oligomers to monomers. Optionally, the hemicellulose oligomers and the hemicelluloses from step (b) are combined and hydrolyzed in a single reactor. The lignin-coated cellulose material contains, on a dry basis, about 3 wt % or less hemicellulose content.

The lignin-coated cellulose material may include one or more materials selected from the group consisting of pulp, dissolving pulp, fibrillated cellulose, microcrystalline cellulose, and nanocellulose. The lignin-coated cellulose material may be combusted as a lignin-rich cellulosic fuel. In some embodiments, the process further comprises recovering, combusting, or further treating the lignin that does not deposit during step (c).

Variations of the invention provide a cellulose-rich composition comprising from about 70 wt % to about 90 wt % cellulose and about 5 wt % to about 15 wt % total lignin, wherein the cellulose-rich composition includes particles with a higher average surface concentration of lignin compared to an average bulk (internal) concentration of lignin.

In some embodiments, the composition comprises from about 75 wt % to about 87 wt % cellulose. In certain embodiments, the composition comprises from about 77 wt % to about 86 wt % cellulose.

In some embodiments, the cellulose-rich composition comprises from about 7 wt % to about 12 wt % lignin. In certain embodiments, the composition comprises from about 8 wt % to about 11 wt % lignin.

In some embodiments, the cellulose-rich composition comprises about 3 wt % or less hemicellulose. In certain embodiments, the composition comprises about 2 wt % or less hemicellulose.

In some embodiments, the composition comprises from about 1.0 wt % to about 2.0 wt % uronic acid groups. In some embodiments, the composition comprises about 2 wt % or less ash.

The cellulose-rich composition may be characterized by an elemental ratio H/(C+H) of about 0.03 to about 0.04, such as about 0.031 to about 0.034, in certain embodiments, where H is total hemicellulose and C is total cellulose.

Various cellulose-containing products may include the cellulose-rich compositions disclosed herein. In various embodiments, the cellulose-containing product is selected from the group consisting of a structural object, a foam, an aerogel, a polymer composite, a carbon composite, a film, a coating, a coating precursor, a current or voltage carrier, a filter, a membrane, a catalyst, a catalyst substrate, a coating additive, a paint additive, an adhesive additive, a cement additive, a paper coating, a thickening agent, a rheological modifier, an additive for a drilling fluid, and combinations or derivatives thereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exemplary block-flow diagram of some embodiments of the invention to fractionate biomass into cellulose, hemicellulose, and lignin, comprising hot-water extraction followed by digestion with an acid and solvent for lignin.

FIG. 2 is an exemplary block-flow diagram depicting a Green Power+® process followed by an AVAP® process to produce lignin-coated cellulose fibers.

FIG. 3 shows photographs of (a) pre-extracted sugarcane straw and (b) lignin-coated cellulose fibers produced, according to some embodiments.

FIG. 4 is a graph of experimental Kappa number versus ethanol concentration in cooking liquor.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

This description will enable one skilled in the art to make and use the invention, and it describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with any accompanying drawings.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly indicates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All composition numbers and ranges based on percentages are weight percentages, unless indicated otherwise. All ranges of numbers or conditions are meant to encompass any specific value contained within the range, rounded to any suitable decimal point.

Unless otherwise indicated, all numbers expressing parameters, reaction conditions, concentrations of components, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending at least upon a specific analytical technique.

The term “comprising,” which is synonymous with “including,” “containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim.

As used herein, the phase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of”.

The present invention, in some variations, is premised on the realization that aspects of Green Power+® and AVAP® technologies may be combined and improved in certain ways to realize various benefits (as described herein).

This disclosure describes processes and apparatus to efficiently fractionate any lignocellulosic-based biomass into its primary major components (cellulose, lignin, and if present, hemicellulose) so that each can be used in potentially distinct processes. An advantage of the process is that it produces cellulose-rich solids while concurrently producing a liquid phase containing a high yield of both hemicellulose sugars and lignin, and low quantities of lignin and hemicellulose degradation products. The flexible fractionation technique enables multiple uses for the products. The cellulose is highly reactive to cellulase enzymes for the manufacture of glucose. Other uses for celluloses can be adjusted based on market conditions.

Certain exemplary embodiments of the invention will now be described. These embodiments are not intended to limit the scope of the invention as claimed. The order of steps may be varied, some steps may be omitted, and/or other steps may be added. Reference herein to first step, second step, etc. is for illustration purposes only.

In some variations, the invention provides a process for producing a lignin-coated cellulose material, the process comprising:

(a) providing a lignocellulosic biomass feedstock;

(b) pre-extracting the feedstock in the presence of steam or hot water, thereby generating a first solids stream and a first liquid stream, wherein the first liquid stream contains hemicelluloses and lignin;

(c) depositing at least some of the lignin, from the first liquid stream, onto a surface of the first solids stream to generate a lignin-coated intermediate material comprising cellulose-rich particles with a lignin coating;

(d) optionally drying the lignin-coated intermediate material;

(e) digesting the lignin-coated intermediate material in the presence of an acid, a solvent for lignin, and water, to generate a second solids stream and a second liquid stream, wherein during the digesting, the rate of delignification of surface lignin deposited from step (c) is lower than the rate of delignification of bulk lignin, thereby retaining at least a portion of the lignin coating; and

(f) recovering the second solids stream as a lignin-coated cellulose material, wherein the lignin-coated cellulose material is at least partially hydrophobic.

In some embodiments, step (b) further includes introducing an acid catalyst to enhance lignin deposition (such as by catalyzing precipitation reactions). Such an acid catalyst may be selected from the group consisting of acetic acid, formic acid, uronic acids, levulinic acid, sulfur dioxide, sulfurous acid, sulfuric acid, lignosulfonic acid, carbon dioxide, carbonic acid, and combinations thereof.

The acid for either step (b), if employed, or step (e) may be derived and recycled from operations downstream. For example, acetic acid may be recycled from evaporator condensate. In some embodiments, a sulfur-containing acid contained in or derived from a liquid stream is recycled to step (b) and/or step (e).

Without being limited by theory, it is believed that the surface lignin, deposited back into the cellulose particles or fibers during pre-extraction, will be chemically more recalcitrant than the native lignin that is present in the bulk, internal portion of the cellulose particles or fibers. It is believed that the surface lignin will experience a slower rate of delignification in the digestion step, and possibly no net delignification, compared to the delignification rate of the bulk lignin. It should be noted that the surface lignin layer is not regarded as an impenetrable layer since bulk lignin and hemicellulose need to diffuse through the layer to enter solution in the digesting step. During digesting, additional lignin may precipitate as well.

In some embodiments, some amount of drying is conducted in step (d). In these or alternative embodiments, water is removed by membranes, molecular sieves, centrifuges, or other means. Drying or other treatment between pre-extraction and digestion may modify the surface lignin so that it is less susceptible to delignification during the digesting step, for example.

In some embodiments, the acid in step (e) is selected from the group consisting of sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, and combinations thereof. The acid is sulfur dioxide, in particular embodiments.

The process may further comprise hydrolyzing the hemicelluloses (in the first liquid stream) to produce monomeric sugars. The second liquid stream also typically contains hemicellulose oligomers; the process may further comprise hydrolyzing the hemicellulose oligomers to monomers. Optionally, the hemicellulose oligomers and the hemicelluloses from step (b) are combined and hydrolyzed in a single reactor. The lignin-coated cellulose material contains, on a dry basis, about 3 wt % or less hemicellulose content.

The lignin-coated cellulose material may include one or more materials selected from the group consisting of pulp, dissolving pulp, fibrillated cellulose, microcrystalline cellulose, and nanocellulose. The lignin-coated cellulose material may be combusted as a lignin-rich cellulosic fuel. In some embodiments, the process further comprises recovering, combusting, or further treating the lignin that does not deposit during step (c).

Variations of the invention provide a cellulose-rich composition comprising from about 70 wt % to about 90 wt % cellulose and about 5 wt % to about 15 wt % total lignin, wherein the cellulose-rich composition includes particles with a higher average surface concentration of lignin compared to an average bulk (internal) concentration of lignin.

In some embodiments, the composition comprises from about 75 wt % to about 87 wt % cellulose. In certain embodiments, the composition comprises from about 77 wt % to about 86 wt % cellulose.

In some embodiments, the cellulose-rich composition comprises from about 7 wt % to about 12 wt % lignin. In certain embodiments, the composition comprises from about 8 wt % to about 11 wt % lignin.

In some embodiments, the cellulose-rich composition comprises about 3 wt % or less hemicellulose. In certain embodiments, the composition comprises about 2 wt % or less hemicellulose.

In some embodiments, the composition comprises from about 1.0 wt % to about 2.0 wt % uronic acid groups. In some embodiments, the composition comprises about 2 wt % or less ash.

The cellulose-rich composition may be characterized by an elemental ratio H/(C+H) of about 0.03 to about 0.04, such as about 0.031 to about 0.034, in certain embodiments, where H is total hemicellulose and C is total cellulose.

Various cellulose-containing products may include the cellulose-rich compositions disclosed herein. In various embodiments, the cellulose-containing product is selected from the group consisting of a structural object, a foam, an aerogel, a polymer composite, a carbon composite, a film, a coating, a coating precursor, a current or voltage carrier, a filter, a membrane, a catalyst, a catalyst substrate, a coating additive, a paint additive, an adhesive additive, a cement additive, a paper coating, a thickening agent, a rheological modifier, an additive for a drilling fluid, and combinations or derivatives thereof.

The second liquid stream typically (although not necessarily) contains hemicellulose oligomers. In some embodiments, the process further comprises hydrolyzing the hemicellulose oligomers to monomers in the presence of heat and optionally a second hydrolysis catalyst. The second hydrolysis catalyst may include an acid selected from the group consisting of acetic acid, formic acid, uronic acids, levulinic acid, sulfur dioxide, sulfurous acid, sulfuric acid, lignosulfonic acid, carbon dioxide, carbonic acid, and combinations thereof.

There are many opportunities for mass and energy integration. In some embodiments, the hemicelluloses from step (e) and the hemicellulose oligomers from step (b) are combined and hydrolyzed in a single reactor (or a series of reactors, tanks, or other units).

In certain embodiments, step (b) is conducted at a first location and step (e) is conducted at a second location that is not co-located at a single site. In these embodiments, the first solids stream is transported (e.g., by truck, rail, barge, or other means) from the first location to the second location, which may be for example 5, 25, 50, 100, 500, 1000 miles away or more.

The biomass feedstock may be selected from hardwoods, softwoods, forest residues, industrial wastes, pulp and paper wastes, consumer wastes, or combinations thereof. Some embodiments utilize agricultural residues, which include lignocellulosic biomass associated with food crops, annual grasses, energy crops, or other annually renewable feedstocks. Exemplary agricultural residues include, but are not limited to, corn stover, corn fiber, wheat straw, sugarcane bagasse, sugarcane straw, rice straw, oat straw, barley straw, miscanthus, energy cane straw/residue, or combinations thereof.

As used herein, “lignocellulosic biomass” means any material containing cellulose and lignin. Lignocellulosic biomass may also contain hemicellulose. Mixtures of one or more types of biomass can be used. In some embodiments, the biomass feedstock comprises both a lignocellulosic component (such as one described above) in addition to a sucrose-containing component (e.g., sugarcane or energy cane) and/or a starch component (e.g., corn, wheat, rice, etc.).

Various moisture levels may be associated with the starting biomass. The biomass feedstock need not be, but may be, relatively dry. In general, the biomass is in the form of a particulate or chip, but particle size is not critical in this invention.

Reaction conditions and operation sequences may vary widely. Some embodiments employ conditions described in U.S. Pat. No. 8,030,039, issued Oct. 4, 2011; U.S. Pat. No. 8,038,842, issued Oct. 11, 2011; U.S. Pat. No. 8,268,125, issued Sep. 18, 2012; U.S. Pat. No. 8,585,863, issued Nov. 19, 2013; and U.S. patent application Ser. Nos. 12/234,286; 13/585,710; 13/626,220; 12/854,869; 12/250,734; 12/397,284; 12/304,046; 13/500,916; 13/626,220; 12/854,869; 14/048,068; 14/005,382; 61/732,047; 61/735,738; 61/747,010; 61/747,105; 61/747,376; 61/747,379; 61/747,382; 61/747,408; 61/747,566; 61/747,771; 61/827,827; 61/845,298; 61/845,046; 61/732,047; 61/739,343; 61/770,130; 61/836,014; and 61/747,631. Each of these commonly owned patent applications is hereby incorporated by reference herein in its entirety. In some embodiments, the process is a variation of the AVAP® process technology which is commonly owned with the assignee of this patent application.

Some embodiments employ conditions described in U.S. Pat. No. 8,211,680, issued Jul. 3, 2012; U.S. Pat. No. 8,518,672, issued Aug. 27, 2013; U.S. Pat. No. 8,518,213, issued Aug. 27, 2013; and U.S. patent application Ser. Nos. 13/950,289; 13/929,858; 12/397,284; 13/471,662; 13/026,273; 13/026,280; 13/500,917; 13/929,858; 13/968,892; 13/829,237; 13/829,355; 13/874,761; 13/959,705; 14/017,286; 14/044,784; 14/044,790; 61/810,767; 61/839,912; and 61/878,421. Each of these commonly owned patent applications is hereby incorporated by reference herein in its entirety. In some embodiments, the process is a variation of the Green Power+® process technology which is commonly owned with the assignee of this patent application.

Some variations may be understood with reference to FIGS. 1 and 2. Dotted lines denote optional streams. Various embodiments will now be further described, without limitation as to the scope of the invention. These embodiments are exemplary in nature.

In some variations associated with FIG. 1, the invention provides a process for fractionating lignocellulosic biomass, the process comprising:

(a) providing a feedstock comprising lignocellulosic biomass;

(b) extracting hemicelluloses from the feedstock in the presence of steam or hot water, and optionally a first hydrolysis catalyst, thereby generating a first solids stream (with lignin-coated cellulose) and a first liquid stream;

(c) contacting the first solids stream with an acid or acid precursor, water, and a solvent for lignin, to produce a second liquid stream containing lignin-coated cellulose-rich solids and lignin;

(d) recovering the lignin-coated cellulose-rich solids from the second liquid stream;

(e) hydrolyzing the hemicelluloses to produce monomeric sugars; and

(f) recovering the monomeric sugars.

In preferred embodiments, the process further comprises recovering or further treating the lignin-coated cellulose-rich solids as pulp, a cellulose product, or a cellulose derivative. In some embodiments, the process further comprises hydrolyzing the lignin-coated cellulose-rich solids using an acid catalyst or cellulase enzymes to produce glucose.

The lignocellulosic material following pre-extraction may be processed in a solution (cooking liquor) of aliphatic alcohol, water, and sulfur dioxide. The cooking liquor preferably contains at least 10 wt %, such as about 20 wt %, 30 wt %, 35 wt % 40 wt %, or 50 wt % of a solvent for lignin. For example, the cooking liquor may contain about 10-70 wt % solvent, such as about 30 wt % solvent. The solvent for lignin may be an aliphatic alcohol, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 1-hexanol, or cyclohexanol. The solvent for lignin may be an aromatic alcohol, such as phenol or cresol. Other lignin solvents are possible, such as (but not limited to) glycerol, methyl ethyl ketone, or diethyl ether. Combinations of more than one solvent may be employed.

Preferably, enough solvent is included in the extractant mixture to dissolve the lignin present in the starting material. The solvent for lignin may be completely miscible, partially miscible, or immiscible with water, so that there may be more than one liquid phase. Potential process advantages arise when the solvent is miscible with water, and also when the solvent is immiscible with water. When the solvent is water-miscible, a single liquid phase forms, so mass transfer of lignin and hemicellulose extraction is enhanced, and the downstream process must only deal with one liquid stream. When the solvent is immiscible in water, the extractant mixture readily separates to form liquid phases, so a distinct separation step can be avoided or simplified. This can be advantageous if one liquid phase contains most of the lignin and the other contains most of the hemicellulose sugars, as this facilitates recovering the lignin from the hemicellulose sugars.

The cooking liquor for the digestor contains an effective amount of an acid or acid precursor. Acids may be sulfur-containing acids (e.g. SO₂ or lignosulfonic acid), nitrogen-containing acids (e.g. nitric acid), phosphorus-containing acids (e.g. phosphoric acid), carbon-containing acids (e.g. carbonic acid), and so on. An “acid precursor” is any compound that, at least in part, forms or releases an acid in the digestor. Sulfur dioxide may be considered an acid precursor since SO₂ is not itself a Brønsted acid, although SO₂ is a Lewis acid.

The cooking liquor preferably contains sulfur dioxide and/or sulfurous acid (H₂SO₃). The cooking liquor preferably contains SO₂, in dissolved or reacted form, in a concentration of at least 3 wt %, preferably at least 6 wt %, more preferably at least 8 wt %, such as about 9 wt %, 10 wt %, 11 wt %, 12 wt %, 13 wt %, 14 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt % or higher. The cooking liquor may also contain one or more species, separately from SO₂, to adjust the pH. The pH of the cooking liquor is typically about 4 or less.

Sulfur dioxide is a preferred acid catalyst, because it can be recovered easily from solution after hydrolysis. The majority of the SO₂ from the hydrolysate may be stripped and recycled back to the reactor. Recovery and recycling translates to less lime required compared to neutralization of comparable sulfuric acid, less solids to dispose of, and less separation equipment. The increased efficiency owing to the inherent properties of sulfur dioxide mean that less total acid or other catalysts may be required. This has cost advantages, since sulfuric acid can be expensive. Additionally, and quite significantly, less acid usage also will translate into lower costs for a base (e.g., lime) to increase the pH following hydrolysis, for downstream operations. Furthermore, less acid and less base will also mean substantially less generation of waste salts (e.g., gypsum) that may otherwise require disposal.

In some embodiments, an additive may be included in amounts of about 0.1 wt % to 10 wt % or more to increase cellulose viscosity. Exemplary additives include ammonia, ammonia hydroxide, urea, anthraquinone, magnesium oxide, magnesium hydroxide, sodium hydroxide, and their derivatives.

The cooking is performed in one or more stages using batch or continuous digestors. Solid and liquid may flow cocurrently or countercurrently, or in any other flow pattern that achieves the desired fractionation. The cooking reactor may be internally agitated, if desired.

Depending on the lignocellulosic material to be processed, the cooking conditions are varied, with temperatures from about 65° C. to 175° C., for example 75° C., 85° C., 95° C., 105° C., 115° C., 125° C., 130° C., 135° C., 140° C., 145° C., 150° C., 155° C., 165° C. or 170° C., and corresponding pressures from about 1 atmosphere to about 15 atmospheres in the liquid or vapor phase. The cooking time of one or more stages may be selected from about 15 minutes to about 720 minutes, such as about 30, 45, 60, 90, 120, 140, 160, 180, 250, 300, 360, 450, 550, 600, or 700 minutes. Generally, there is an inverse relationship between the temperature used during the digestion step and the time needed to obtain good fractionation of the biomass into its constituent parts.

The cooking liquor to lignocellulosic material ratio may be selected from about 1 to about 10, such as about 2, 3, 4, 5, or 6. In some embodiments, biomass is digested in a pressurized vessel with low liquor volume (low ratio of cooking liquor to lignocellulosic material), so that the cooking space is filled with ethanol and sulfur dioxide vapor in equilibrium with moisture. The cooked biomass is washed in alcohol-rich solution to recover lignin and dissolved hemicelluloses, while the remaining pulp is further processed. In some embodiments, the process of fractionating lignocellulosic material comprises vapor-phase cooking of lignocellulosic material with aliphatic alcohol (or other solvent for lignin), water, and sulfur dioxide. See, for example, U.S. Pat. Nos. 8,038,842 and 8,268,125 which are incorporated by reference herein.

A portion or all of the sulfur dioxide may be present as sulfurous acid in the extract liquor. In certain embodiments, sulfur dioxide is generated in situ by introducing sulfurous acid, sulfite ions, bisulfate ions, combinations thereof, or a salt of any of the foregoing. Excess sulfur dioxide, following hydrolysis, may be recovered and reused.

In some embodiments, sulfur dioxide is saturated in water (or aqueous solution, optionally with an alcohol) at a first temperature, and the hydrolysis is then carried out at a second, generally higher, temperature. In some embodiments, sulfur dioxide is sub-saturated. In some embodiments, sulfur dioxide is super-saturated. In some embodiments, sulfur dioxide concentration is selected to achieve a certain degree of lignin sulfonation, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% sulfur content. SO₂ reacts chemically with lignin to form stable lignosulfonic acids which may be present both in the solid and liquid phases.

The concentration of sulfur dioxide, additives, and aliphatic alcohol (or other solvent) in the solution and the time of cook may be varied to control the yield of cellulose and hemicellulose in the pulp. The concentration of sulfur dioxide and the time of cook may be varied to control the yield of lignin versus lignosulfonates in the hydrolysate. In some embodiments, the concentration of sulfur dioxide, temperature, and the time of cook may be varied to control the yield of fermentable sugars.

Once the desired amount of fractionation of both hemicellulose and lignin from the solid phase is achieved, the liquid and solid phases are separated. Conditions for the separation may be selected to minimize the reprecipitation of the extracted lignin on the solid phase. This is favored by conducting separation or washing at a temperature of at least the glass-transition temperature of lignin (about 120° C.).

The physical separation can be accomplished either by transferring the entire mixture to a device that can carry out the separation and washing, or by removing only one of the phases from the reactor while keeping the other phase in place. The solid phase can be physically retained by appropriately sized screens through which liquid can pass. The solid is retained on the screens and can be kept there for successive solid-wash cycles. Alternately, the liquid may be retained and solid phase forced out of the reaction zone, with centrifugal or other forces that can effectively transfer the solids out of the slurry. In a continuous system, countercurrent flow of solids and liquid can accomplish the physical separation.

The recovered solids normally will contain a quantity of lignin and sugars, some of which can be removed easily by washing. The washing-liquid composition can be the same as or different than the liquor composition used during fractionation. Multiple washes may be performed to increase effectiveness. Preferably, one or more washes are performed with a composition including a solvent for lignin, to remove additional lignin from the solids, followed by one or more washes with water to displace residual solvent and sugars from the solids. Recycle streams, such as from solvent-recovery operations, may be used to wash the solids.

After separation and washing as described, a solid phase and at least one liquid phase are obtained. The solid phase contains substantially undigested cellulose. A single liquid phase is usually obtained when the solvent and the water are miscible in the relative proportions that are present. In that case, the liquid phase contains, in dissolved form, most of the lignin originally in the starting lignocellulosic material, as well as soluble monomeric and oligomeric sugars formed in the hydrolysis of any hemicellulose that may have been present. Multiple liquid phases tend to form when the solvent and water are wholly or partially immiscible. The lignin tends to be contained in the liquid phase that contains most of the solvent. Hemicellulose hydrolysis products tend to be present in the liquid phase that contains most of the water.

In some embodiments, hydrolysate from the cooking step is subjected to pressure reduction. Pressure reduction may be done at the end of a cook in a batch digestor, or in an external flash tank after extraction from a continuous digestor, for example. The flash vapor from the pressure reduction may be collected into a cooking liquor make-up vessel. The flash vapor contains substantially all the unreacted sulfur dioxide which may be directly dissolved into new cooking liquor. The cellulose is then removed to be washed and further treated as desired.

A process washing step recovers the hydrolysate from the cellulose. The washed cellulose is pulp that may be used for various purposes (e.g., paper or nanocellulose production). The weak hydrolysate from the washer continues to the final reaction step; in a continuous digestor this weak hydrolysate may be combined with the extracted hydrolysate from the external flash tank. In some embodiments, washing and/or separation of hydrolysate and cellulose-rich solids is conducted at a temperature of at least about 100° C., 110° C., or 120° C. The washed cellulose may also be used for glucose production via cellulose hydrolysis with enzymes or acids.

In another reaction step, the hydrolysate may be further treated in one or multiple steps to hydrolyze the oligomers into monomers. This step may be conducted before, during, or after the removal of solvent and sulfur dioxide. The solution may or may not contain residual solvent (e.g. alcohol). In some embodiments, sulfur dioxide is added or allowed to pass through to this step, to assist hydrolysis. In these or other embodiments, an acid such as sulfurous acid or sulfuric acid is introduced to assist with hydrolysis. In some embodiments, the hydrolysate is autohydrolyzed by heating under pressure. In some embodiments, no additional acid is introduced, but lignosulfonic acids produced during the initial cooking are effective to catalyze hydrolysis of hemicellulose oligomers to monomers. In various embodiments, this step utilizes sulfur dioxide, sulfurous acid, sulfuric acid at a concentration of about 0.01 wt % to 30 wt %, such as about 0.05 wt %, 0.1 wt %, 0.2 wt %, 0.5 wt %, 1 wt %, 2 wt %, 5 wt %, 10 wt %, or 20 wt %. This step may be carried out at a temperature from about 100° C. to 220° C., such as about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., or 210° C. Heating may be direct or indirect to reach the selected temperature.

The reaction step produces fermentable sugars which can then be concentrated by evaporation to a fermentation feedstock. Concentration by evaporation may be accomplished before, during, or after the treatment to hydrolyze oligomers. The final reaction step may optionally be followed by steam stripping of the resulting hydrolysate to remove and recover sulfur dioxide and alcohol, and for removal of potential fermentation-inhibiting side products. The evaporation process may be under vacuum or pressure, from about −0.1 atmospheres to about 10 atmospheres, such as about 0.1 atm, 0.3 atm, 0.5 atm, 1.0 atm, 1.5 atm, 2 atm, 4 atm, 6 atm, or 8 atm.

Recovering and recycling the sulfur dioxide may utilize separations such as, but not limited to, vapor-liquid disengagement (e.g. flashing), steam stripping, extraction, or combinations or multiple stages thereof. Various recycle ratios may be practiced, such as about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, or more. In some embodiments, about 90-99% of initially charged SO₂ is readily recovered by distillation from the liquid phase, with the remaining 1-10% (e.g., about 3-5%) of the SO₂ primarily bound to dissolved lignin in the form of lignosulfonates.

In a preferred embodiment, the evaporation step utilizes an integrated alcohol stripper and evaporator. Evaporated vapor streams may be segregated so as to have different concentrations of organic compounds in different streams. Evaporator condensate streams may be segregated so as to have different concentrations of organic compounds in different streams. Alcohol may be recovered from the evaporation process by condensing the exhaust vapor and returning to the cooking liquor make-up vessel in the cooking step. Clean condensate from the evaporation process may be used in the washing step.

In some embodiments, an integrated alcohol stripper and evaporator system is employed, wherein aliphatic alcohol is removed by vapor stripping, the resulting stripper product stream is concentrated by evaporating water from the stream, and evaporated vapor is compressed using vapor compression and is reused to provide thermal energy.

The hydrolysate from the evaporation and final reaction step contains mainly fermentable sugars but may also contain lignin depending on the location of lignin separation in the overall process configuration. The hydrolysate may be concentrated to a concentration of about 5 wt % to about 60 wt % solids, such as about 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %, 45 wt %, 50 wt % or 55 wt % solids. The hydrolysate contains fermentable sugars.

Fermentable sugars are defined as hydrolysis products of cellulose, galactoglucomannan, glucomannan, arabinoglucuronoxylans, arabinogalactan, and glucuronoxylans into their respective short-chained oligomers and monomer products, i.e., glucose, mannose, galactose, xylose, and arabinose. The fermentable sugars may be recovered in purified form, as a sugar slurry or dry sugar solids, for example. Any known technique may be employed to recover a slurry of sugars or to dry the solution to produce dry sugar solids.

In some embodiments, the fermentable sugars are fermented to produce biochemicals or biofuels such as (but by no means limited to) ethanol, isopropanol, acetone, 1-butanol, isobutanol, lactic acid, succinic acid, or any other fermentation products. Some amount of the fermentation product may be a microorganism or enzymes, which may be recovered if desired.

When the fermentation will employ bacteria, such as Clostridia bacteria, it is preferable to further condition the hydrolysate to raise pH and remove residual SO₂ and other fermentation inhibitors. The residual SO₂ (i.e., following removal of most of it by stripping or other means) may be catalytically oxidized to convert residual sulfite ions to sulfate ions by oxidation. This oxidation may be accomplished by adding an oxidation catalyst, such as FeSO4.7H₂O, that oxidizes sulfite ions to sulfate ions. Preferably, the residual SO₂ is reduced to less than about 100 ppm, 50 ppm, 25 ppm, 10 ppm, 5 ppm, or 1 ppm.

In some embodiments, the process further comprises recovering the lignin as a co-product. The sulfonated lignin may also be recovered as a co-product. In certain embodiments, the process further comprises combusting or gasifying the sulfonated lignin, recovering sulfur contained in the sulfonated lignin in a gas stream comprising reclaimed sulfur dioxide, and then recycling the reclaimed sulfur dioxide for reuse.

The process lignin separation step is for the separation of lignin from the hydrolysate and can be located before or after the final reaction step and evaporation. If located after, then lignin will precipitate from the hydrolysate since alcohol has been removed in the evaporation step. The remaining water-soluble lignosulfonates may be precipitated by converting the hydrolysate to an alkaline condition (pH higher than 7) using, for example, an alkaline earth oxide, preferably calcium oxide (lime). The combined lignin and lignosulfonate precipitate may be filtered. The lignin and lignosulfonate filter cake may be dried as a co-product or burned or gasified for energy production. The hydrolysate from filtering may be recovered and sold as a concentrated sugar solution product or further processed in a subsequent fermentation or other reaction step.

Native (non-sulfonated) lignin is hydrophobic, while lignosulfonates are hydrophilic. Hydrophilic lignosulfonates may have less propensity to clump, agglomerate, and stick to surfaces. Even lignosulfonates that do undergo some condensation and increase of molecular weight, will still have an HSO₃ group that will contribute some solubility (hydrophilic).

In some embodiments, the soluble lignin precipitates from the hydrolysate after solvent has been removed in the evaporation step. In some embodiments, reactive lignosulfonates are selectively precipitated from hydrolysate using excess lime (or other base, such as ammonia) in the presence of aliphatic alcohol. In some embodiments, hydrated lime is used to precipitate lignosulfonates. In some embodiments, part of the lignin is precipitated in reactive form and the remaining lignin is sulfonated in water-soluble form.

The process fermentation and distillation steps are intended for the production of fermentation products, such as alcohols or organic acids. After removal of cooking chemicals and lignin, and further treatment (oligomer hydrolysis), the hydrolysate contains mainly fermentable sugars in water solution from which any fermentation inhibitors have been preferably removed or neutralized. The hydrolysate is fermented to produce dilute alcohol or organic acids, from 1 wt % to 20 wt % concentration. The dilute product is distilled or otherwise purified as is known in the art.

When alcohol is produced, such as ethanol, some of it may be used for cooking liquor makeup in the process cooking step. Also, in some embodiments, a distillation column stream, such as the bottoms, with or without evaporator condensate, may be reused to wash cellulose. In some embodiments, lime may be used to dehydrate product alcohol. Side products may be removed and recovered from the hydrolysate. These side products may be isolated by processing the vent from the final reaction step and/or the condensate from the evaporation step. Side products include furfural, hydroxymethylfurfural (HMF), methanol, acetic acid, and lignin-derived compounds, for example.

When hemicellulose is present in the starting biomass, all or a portion of the liquid phase contains hemicellulose sugars and soluble oligomers. It is preferred to remove most of the lignin from the liquid, as described above, to produce a fermentation broth which will contain water, possibly some of the solvent for lignin, hemicellulose sugars, and various minor components from the digestion process. This fermentation broth can be used directly, combined with one or more other fermentation streams, or further treated. Further treatment can include sugar concentration by evaporation; addition of glucose or other sugars (optionally as obtained from cellulose saccharification); addition of various nutrients such as salts, vitamins, or trace elements; pH adjustment; and removal of fermentation inhibitors such as acetic acid and phenolic compounds. The choice of conditioning steps should be specific to the target product(s) and microorganism(s) employed.

In some embodiments, hemicellulose sugars are not fermented but rather are recovered and purified, stored, sold, or converted to a specialty product. Xylose, for example, may be converted into xylitol.

A lignin product can be readily obtained from a liquid phase using one or more of several methods. One simple technique is to evaporate off all liquid, resulting in a solid lignin-rich residue. This technique would be especially advantageous if the solvent for lignin is water-immiscible. Another method is to cause the lignin to precipitate out of solution. Some of the ways to precipitate the lignin include (1) removing the solvent for lignin from the liquid phase, but not the water, such as by selectively evaporating the solvent from the liquid phase until the lignin is no longer soluble; (2) diluting the liquid phase with water until the lignin is no longer soluble; and (3) adjusting the temperature and/or pH of the liquid phase. Methods such as centrifugation can then be utilized to capture the lignin. Yet another technique for removing the lignin is continuous liquid-liquid extraction to selectively remove the lignin from the liquid phase, followed by removal of the extraction solvent to recover relatively pure lignin.

Lignin produced in accordance with the invention can be used as a fuel. As a solid fuel, lignin is similar in energy content to coal. Lignin can act as an oxygenated component in liquid fuels, to enhance octane while meeting standards as a renewable fuel. The lignin produced herein can also be used as polymeric material, and as a chemical precursor for producing lignin derivatives. The sulfonated lignin may be sold as a lignosulfonate product, or burned for fuel value.

The present invention also provides systems configured for carrying out the disclosed processes, and compositions produced therefrom. Any stream generated by the disclosed processes may be partially or completed recovered, purified or further treated, analyzed (including on-line or off-line analysis), and/or marketed or sold.

In this detailed description, reference has been made to multiple embodiments of the invention and non-limiting examples relating to how the invention can be understood and practiced. Other embodiments that do not provide all of the features and advantages set forth herein may be utilized, without departing from the spirit and scope of the present invention. This invention incorporates routine experimentation and optimization of the methods and systems described herein. Such modifications and variations are considered to be within the scope of the invention defined by the claims.

All publications, patents, and patent applications cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein.

Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially.

Therefore, to the extent there are variations of the invention, which are within the spirit of the disclosure or equivalent to the inventions found in the appended claims, it is the intent that this patent will cover those variations as well. The present invention shall only be limited by what is claimed.

Examples Production of Lignin-Coated Cellulose Fibers by Implementation of AVAP Process Following Hot Water Pre-Extraction of Lignocellulosic Biomass by Green Power+ Process

Green Power+ pre-extraction of sugarcane straws with water was performed at 180° C. for 2.5 and 10 min at L/W ratio of 4:1. The extraction yields of 2.5 and 10 min hot water pre-treatments were 28 and 35% respectively. Hemicellulose and mineral removal of the straws were promoted with hot water pretreatment by Green Power+ process addition prior to AVAP process; lignin condensation on cellulose fibers took place which hindered delignification during AVAP cooking. Therefore, lignin-coated cellulose fibers with low hemicellulose and lignin contents can be produced by AVAP process following the hot water pre-extraction of lignocellulosic biomass by Green Power+.

Material and Methods

Sugar cane straws were used as lignocellulosic feedstock for the experiments. Green Power+ pre-treatment and AVAP cooking of the straws were conducted in mini-reactors of multi digester oil bath (MDOB). Subsequent fiber washing with 40% ethanol following the AVAP process was conducted and cellulose fibers were prepared after defibrillation, disintegration and screening processes.

Subsequent Fiber Washing Procedure Following AVAP Process

The mini-reactors were rapidly removed from the oil bath at the end of AVAP cooking and they were placed in an ice-water bath. The bombs were opened after about 10 min of cooling. The cooking mixture (the fibers and liquor) was separated using a nylon washing bag by squeezing. The nylon bag was then put into a plastic bag and 50 ml of washing liquid (40% (w/w) ethanol) was added into the plastic bag. The EtOH washing was conducted in a water bath at 60° C. for 5 min. Liquor is squeezed out after the 1^(st) EtOH washing. The washing step was repeated one more time for more complete washing. The nylon bag was placed into a 1 L beaker and it was washed with 500 ml of water at 20° C. for 5 min. The water washing step was repeated one more time. Finally, the cellulose fibers were mixed with about 500 ml of water and stirred about 5 min with a mechanical stirrer and filtered through the Whatman filter paper to produce the fiber pad (cellulose fiber).

Composition Analysis of Straws and Fibers

Sugar cane straws and cellulose fibers were air dried and ground to less than 0.1 mm using a Wiley Mill and the moisture content of the milled particles was determined using a convection oven at 100±5° C. overnight. The ash content of the samples was determined according to Tappi standard method T211 om-85. The extractives content of air dried ground particles was determined by the Soxhlet extraction method with acetone. The acid insoluble lignin content, or Klason lignin, was determined according to the method by Effland (1977), while the acid soluble lignin content was determined by Tappi Method 250. The uronic anhydride content was determined using the chromophoric group analysis method developed by Scott (1979). The milled straw particles were first acid hydrolyzed with 72% H₂SO₄ in a water bath at 30±1° C. for 2 hours and then exposed to secondary acid hydrolysis at 4% H₂SO₄ in an autoclave at 120±1° C. and 2 hours for monosugar content analysis (Davis, 1998). A High Performance Anion Exchange Chromatography with Pulse Amperometric Detection (HPAEC-PAD) was used for separation of the monosugars. Acetic acid in the hydrolysate was determined by High Performance Liquid Chromatography (HPLC) using a refractive index detector and BIO-RAD Aminex HPX-87H column. The mobile phase used was 0.6 mL/min 5 mM H₂SO₄, and the oven temperature was 60° C. The Kappa number of the fibers was analyzed by Tappi method T236 om-99.

Results and Discussion

The chemical composition of the original sugar cane straw used for the pre-extraction experiments is summarized in Table 1. The cellulose and hemicellulose contents are 34.9±0.6 and 23.9±0.8% respectively based on oven dried (od) straw.

TABLE 1 The chemical composition of sugar cane straws based on od straw Component % Component % Arabinan 3.0 ± 0.6 OAc^(a) 0.9 ± 0.0 Galactan 1.1 ± 0.0 UAG^(b) 1.4 ± 0.0 Glucan 35.3 ± 0.7  Lignin 20.5 ± 0.4  Xylan 18.6 ± 0.5  Extractives 3.1 ± 0.0 Mannan 0.7 ± 0.5 Ash 3.1 ± 0.0 ^(a)Acetyl groups, ^(b)Uronic acid groups

Hot water pre-extraction of sugar cane straws by Green Power+ was performed in the mini reactors of MDOB at 180° C. for 2.5 and 10 min. The liquid to wood ratio of the pre-extraction was 4:1. The sugar cane straws subjected to hot water pre-extraction were washed in total with about 950 ml of water. First the straws were rinsed with 9 times with 50 ml of water (collecting 450 ml of rinsing water identified as 1^(st) wash), and then the straws were washed with 500 ml water (2^(nd) wash) for about 30 min with occasional stirring. The extraction yields following 2.5 and 10 min of pre-extraction were 28 and 35% respectively. Solid phases following hot water pre-extractions were dried in a convection oven overnight at 100±5° C. and oven dried solids were subjected to AVAP cooking at 155° C. for 58 min with 12% SO₂ in 50% (w/w) EtOH at L/W ratio of 4:1 as illustrated in FIG. 2.

Yield and Kappa number of fibers produced from AVAP cooking of hot-water pre-extracted sugar cane straws are summarized in Table 3. It is apparent from Table 3 that the fiber yield increases with addition of hot water pre-extraction. The Kappa number is also increased with hot water pre-extraction. The longer the hot water pre-extraction, the higher the yield and Kappa number. The reason for such high a Kappa number with hot water pre-extraction addition is believed to be that the lignin remaining after hot water pre-extraction is more condensed (Iakovlev, 2011) and that dissolved lignin is re-deposited (Xu et al., 2007) on cellulose fibers due to condensation reactions. Indirect evidence for condensation of lignin is the dark brown color of the hot water pre-extracted straws and cellulose fibers produced from hot water pre-extracted straws with AVAP cooking as shown in FIG. 3. The lignin-free yields as a result of 2.5 min and 10 min hot water addition are around 36 and 37% respectively. The lignin-free yield without any hot water extraction is around 35%.

TABLE 3 Results of AVAP cooking following hot water pre-extraction of straws Pre-extraction with Pre-extraction with No GP⁺ process (2.5 min) GP⁺ process (10 min) Pre-extraction Yield (%) 44.3 47.6 38.5 Kappa # 56.1 68.8 23.8 pH ~0.76 ~0.76 ~1.0

FIG. 3 shows a picture of the (a) pre-extracted sugar cane straws and (b) cellulose fiber produced by the AVAP process (starting with the pre-extracted sugar cane straws).

The composition of the cellulose fibers produced from hot water pre-extracted sugar cane straws by the AVAP process at 155° C. for 58 min at L/W ratio of 4:1 with 12% SO₂ in 50% EtOH is summarized in Table 4. It is clear from Table 4 that the hemicellulose content of fibers produced with the AVAP process may be reduced by addition of the hot water pre-treatment as well as that the hemicellulose content to relative cellulose and hemicellulose content can be improved with hot water pre-extraction at 180° C. for 2.5 and 10 min. As was discussed earlier, condensation of lignin takes place during hot water pre-extraction and hinders delignification during AVAP process (see Table 4). Although all acetyl groups are removed during AVAP cooking of sugar cane straws with or without addition of the Green Power+ process, the cellulose fibers produced from hot water pre-treated straws still have higher uronic acid groups. Somehow hot water pre-extraction addition obstructs dissolution of uronic acid groups of sugar cane straws during AVAP process. It is clear from Table 4 that hot water pre-treatment helps removal of minerals from sugar cane straw that might diminish the acid neutralization power of minerals in the straws during AVAP cooking. Indirect evidence of this is the lower pH of the extract generated during AVAP cooking of hot water pre-extracted straws (see Table 3).

TABLE 4 Composition of cellulose fibers produced by AVAP process following hot water pre-extraction of the straws GP⁺ Process Cell Hemi Lignin OAc UAG Ash H/(C + H), % 2.5 min 84.1 ± 1.5 2.9 ± 0.1 8.5 0.0 1.7 ± 0.1 1.6 ± 0.1 3.3  10 min 78.6 ± 1.1 2.6 ± 0.1 10.5 0.0 1.4 ± 0.1 1.5 ± 0.1 3.2 None 88.1 ± 0.1 3.7 ± 0.1 3.2 0.0 0.8 ± 0.2 2.9 ± 0.4 4.2 C: Cellulose; H: Hemicellulose; OAc: Acetyl Groups; UAG: Uronic Acid Groups

Conclusions

Hot water pre-extraction of sugarcane straws was performed in the MDOB at 180° C. for 2.5 and 10 min at a L/W ratio of 4:1 and the extraction yields were 28 and 35% for 2.5 and 10 min respectively. Although hemicellulose and mineral removal from sugar cane straws were promoted with hot water pretreatment prior to AVAP cooking, condensation of lignin takes place during pre-treatment and hinders subsequent delignification during AVAP process. These combined process can be implemented for production of novel lignin-coated cellulose fibers.

Effect of Ethanol (EtOH) Concentration on AVAP Cooking of Sugar Cane Straws

AVAP cooking of sugar cane straws conducted with lower EtOH concentration (35% (w/w)) in MDOB was compared with previously conducted AVAP cooking at higher EtOH concentration (50% (w/w)). Both cooks were performed at 155° C. for 58 minutes with 12% SO₂ at L/W ratio of 4 L/Kg and results are summarized in Table 5.

It is apparent from Table 5 that the fiber yield as a result of AVAP cooking with 35% EtOH is higher than that of with 50% EtOH because more lignin dissolved at higher EtOH content as can easily be inferred from lower Kappa number of the cellulose fibers produced at 50% EtOH cooking.

As was expected and shown in Table 4, the extract generated as a result of AVAP cooking at 35% EtOH has a higher acidity (lower pH) due to the lower EtOH concentration. The relatively high Kappa number produced at lower EtOH (35%) concentration is probably due to both low EtOH concentration and higher acidity (see Table 5). It is well known that ethanol is a much better solvent for lignin and lignosulfonates than water (Primakov et al. 1979), and higher acidity can cause condensation of lignin during AVAP cooking (Iakovlev, 2011; Sierra-Alvarez and Tjeerdsma, 1995; Eliashberg et al., 1960; Primakov 1961a).

Lignin-free yields of fiber produced with 35 and 50% EtOH are 38 and 35% respectively. This is interesting since acidity during 35% EtOH cooking is higher than that of 50% EtOH cooking (see Table 7); higher dissolution yield of non-lignin straw components is expected during 35% EtOH cooking. The possible explanation is that hemicelluloses are dissolved along with lignin as lignin carbohydrate complexes (Fengel and Wegener, 1984) during 50% EtOH cooking since more lignin dissolved during the higher EtOH AVAP cook as mentioned earlier.

TABLE 5 Results of AVAP cooking of the straw with 35 and 50% EtOH 35% EtOH 50% EtOH Yield (%) 44.1 ± 1.2 38.5 ± 0.6 Kappa # 31.6 ± 0.9 23.8 ± 1.7 Highest Kappa 32.5 25.0 Lowest Kappa 31.3 22.5 pH ~0.60 ~1.0 Number of cooks 2 2

The composition of cellulose fibers produced by AVAP cooking of sugar cane straws at 155° C. for 58 minutes at L/W ratio of 4 L/Kg with 12% SO₂ in 35 and 50% EtOH are summarized in Table 6. It is clear from Table 6 that the hemicellulose content of fibers produced with AVAP cooking could be reduced with decreasing EtOH concentration of the AVAP cooking liquor. Higher acidity of AVAP cooking as a result of lower EtOH concentration promotes hemicellulose removal (Iakovlev, 2011). However the selectivity of hemicellulose removal relative to total carbohydrate (cellulose and hemicellulose) is only slightly improved by reducing the EtOH concentration since the cellulose content of the fibers produced at higher EtOH concentration is higher than that for the lower EtOH concentration cook. Both fibers produced during AVAP cooking of sugar cane straws with 35 and 50% EtOH are completely deacetylated, while also the uronic acid groups and ash content of these two fibers are the same.

TABLE 6 Composition of cellulose fibers produced by the AVAP process with different EtOH concentration ID Cell Hemi Lignin AcG UAG Ash Hemicellulose 35% EtOH 78.0 ± 0.8 3.9 ± 0.2 4.8 ± 0.1 0.0 1.5 ± 0.3 8.5 ± 0.4 4.7 50% EtOH 82.3 ± 1.3 4.5 ± 0.1 3.5 ± 0.3 0.0 1.6 ± 0.3 8.4 ± 0.1 5.2 C: Cellulose; H: Hemicellulose; AcG: Acetyl Groups; UAG: Uronic Acid Groups

Similarly, AVAP cooking of sugar cane straws were conducted at 155 for 58 min with 12% SO₂ at the L/W ratio of 4. Fibers were subjected to subsequent fiber washing twice with 50 g of 60% ethanol at 60 C for 30 min following the AVAP cooks. FIG. 4 shows the Kappa number of straw pulp produced by AVAP versus ethanol ratio (percentage) of AVAP cooking liquor. It is clear from FIG. 4 that lignin content of pulp decreases with increasing ethanol concentration up to 30%, then stays constant in the range of study.

Effect of Liquid to Wood Ratio on AVAP Cooking of Sugar Cane Straws

Sugar cane straws were subjected to AVAP cooking in the Multi Digester Oil Bath (MDOB) at 155° C. for 58 minutes with 12% SO₂ dissolved in 50% (w/w) EtOH solution at different L/W ratios (from 3 to 6). Cellulose fibers without any rejects were produced during AVAP cooking of sugarcane straws at the conditions summarized above. Cellulose fiber yield was calculated after AVAP cooking of the straw and the Kappa number of the fibers produced was determined and summarized in Table 7. The pH values of AVAP extracts were recorded and are also listed in Table 7. It is apparent from Table 7 that Kappa number of pulp (cellulose fiber) produced with lower L/W ratio (3) is higher than with that of higher L/W ratio (6).

TABLE 7 Results of AVAP cooking of the straw at LW ratio of 3 and 6 L/W = 3:1 L/W = 6:1 Yield (%) 41.6 ± 1.0 39.9 ± 0.1 Kappa # 33.9 ± 3.9 23.9 ± 0.5 Highest Kappa 39.1 24.4 Lowest Kappa 29.9 23.5 pH ~0.80 ~0.74 Number of cooks 4 3

The chemical composition of the fibers produced with the AVAP process at two different L/W ratios is summarized in Table 8. Table 8 shows that dissolution of hemicellulose and lignin increases with increasing L/W ratio. Although both fibers produced with AVAP process at L/W ratio of 3 and 6 contain about 1% uronic acid groups, they are completely deacetylated—indicative of that the covalent bond between uronic acid groups and hemicelluloses are more resistant than that of acetyl groups and hemicellulose against acid hydrolysis (acidity arising mostly from lignosulfonic acids formed during SO₂-EtOH cooking). It is clear from Table 8 that selectivity of cellulose content of fiber relative to total carbohydrates (cellulose and hemicellulose) is improved with increasing L/W ratio during AVAP cooking of sugar cane straws at 155° C. for 58 minutes with 12% SO₂ in 50% (w/w) EtOH.

TABLE 8 Composition of cellulose fibers produced by the AVAP process at different L/W ratio ID Cell Hemi Lignin AcG UAG Ash Hemicellulose L/W:3:1 73.6 ± 0.7 7.5 ± 0.3 5.1 ± 0.6 0.0 1.1 ± 0.2 7.1 ± 0.2 9.2 L/W:6:1 75.5 ± 0.7 5.2 ± 0.5 3.5 ± 0.1 0.0 1.2 ± 0.1 7.5 ± 0.3 6.5 C: Cellulose; H: Hemicellulose; AcG: Acetyl Groups; UAG: Uronic Acid Groups

REFERENCES

-   Davis, M. W., “A Rapid Modified Method for Compositional     Carbohydrate Analysis of Lignocellulosics by High pH Anion-Exchange     Chromatography with Pulsed Amperometric Detection (HPAEC/PAD),” J.     Wood Chemistry and Technology, 18(2): 235-252 (1998) -   Effland, M. J., “Modified procedure to determine acid-insoluble     lignin in wood and pulp,” Tappi 60(10): 143-144 (1977) -   Eliashberg, M. G., Parfenova, A. I., Primakov, S. F., “The     delignification of wood with SO2 solutions not containing     bisulfite,” Trudy Leningrad. Lesotekh. Akad. im. S. M. Kirova     91(2):235-245. (1960) -   Fengel, D. and Wegener, G., Wood: Chemistry, Ultrastructure,     Reactions, Berlin, 1984, Walter de Gruyter -   Iakovlev, M., “SO₂-Ethanol-Water (SEW) Fractionation of     Lignocellulosics,” Doctoral Dissertation, Aalto University (2011) -   Primakov, S. F., “Delignification of larch with water-alcohol     solutions of sulfur dioxide. Nauchn. Tr. Vses. Nauchn.-Issled.”     Inst. Tsellyulozn.-Bumazhn. Prom. 46:83-118. (1961) -   Primakov, S. F., Tsarenko, I. M., Zaplatin, V. P. “Delignification     of wood by aqueous alcohol solutions of sulfur dioxide,” Koksnes     Kimija 6:39-42. (1979) -   Scott, R. W., “Colorimetric determination of hexuronic acids in     plant materials,” Analytical Chemistry 51(7): 936-941 (1979) -   Sierra-Alvarez, R., Tjeerdsma, B., “Organosolv pulping of juvenile     poplar wood.” International Symposium on Wood and Pulping Chemistry,     8th, Helsinki, Jun. 6-9 1995, 2, pp. 207-211 -   Tappi Standard Methods UM 250 “Acid-insoluble lignin in wood and     pulp,” TAPPI Press, Atlanta, Ga., USA (1985) -   Tappi Standard Methods T211 om-85 “Ash in wood, pulp, paper, and     paper board,” TAPPI Press, Atlanta, Ga., USA (1985) -   Tappi Standard Methods T236 om-99 “Kappa number of pulp,” TAPPI     Press, Atlanta, Ga., (1999) -   Xu, Y., Li, K., Zhang, M., “Lignin precipitation on the pulp fibers     in the ethanol-based organosolv pulping,” Colloids and Surfaces A:     Physicochem. Eng. Aspects, 301, 255-263 (2007) 

What is claimed is:
 1. A process for producing a lignin-coated cellulose material, said process comprising: (a) providing a lignocellulosic biomass feedstock; (b) pre-extracting said feedstock in the presence of steam or hot water, thereby generating a first solids stream and a first liquid stream, wherein said first liquid stream contains hemicelluloses and lignin; (c) depositing at least some of said lignin, from said first liquid stream, onto a surface of said first solids stream to generate a lignin-coated intermediate material comprising cellulose-rich particles with a lignin coating; (d) optionally drying said lignin-coated intermediate material; (e) digesting said lignin-coated intermediate material in the presence of an acid, a solvent for lignin, and water, to generate a second solids stream and a second liquid stream, wherein during said digesting, the rate of delignification of surface lignin deposited from step (c) is lower than the rate of delignification of bulk lignin, thereby retaining at least a portion of said lignin coating; and (f) recovering said second solids stream as a lignin-coated cellulose material, wherein said lignin-coated cellulose material is at least partially hydrophobic.
 2. The process of claim 1, wherein step (b) further includes introducing an acid catalyst to enhance lignin deposition.
 3. The process of claim 2, wherein said acid catalyst includes an acid selected from the group consisting of acetic acid, formic acid, uronic acids, levulinic acid, sulfur dioxide, sulfurous acid, sulfuric acid, lignosulfonic acid, carbon dioxide, carbonic acid, and combinations thereof.
 4. The process of claim 1, wherein said drying is conducted in step (d).
 5. The process of claim 1, wherein said acid in step (e) is selected from the group consisting of sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, and combinations thereof.
 6. The process of claim 5, wherein said acid is sulfur dioxide.
 7. The process of claim 1, said process further comprising hydrolyzing said hemicelluloses to produce monomeric sugars.
 8. The process of claim 1, wherein said second liquid stream contains hemicellulose oligomers; said process further comprising hydrolyzing said hemicellulose oligomers to monomers.
 9. The process of claim 8, wherein said hemicellulose oligomers and said hemicelluloses from step (b) are combined and hydrolyzed in a single reactor.
 10. The process of claim 1, wherein said lignin-coated cellulose material includes one or more materials selected from the group consisting of pulp, dissolving pulp, fibrillated cellulose, microcrystalline cellulose, and nanocellulose.
 11. The process of claim 1, wherein said lignin-coated cellulose material is combusted as a lignin-rich cellulosic fuel.
 12. The process of claim 1, said process further comprising recovering, combusting, or further treating said lignin that does not deposit during step (c).
 13. The process of claim 1, wherein said lignin-coated cellulose material contains, on a dry basis, about 3 wt % or less hemicellulose content.
 14. A cellulose-rich composition comprising from about 70 wt % to about 90 wt % cellulose and about 5 wt % to about 15 wt % total lignin, wherein said cellulose-rich composition includes particles with a higher average surface concentration of lignin compared to an average bulk (internal) concentration of lignin.
 15. The cellulose-rich composition of claim 14, wherein said composition comprises from about 75 wt % to about 87 wt % cellulose.
 16. The cellulose-rich composition of claim 14, wherein said composition comprises from about 7 wt % to about 12 wt % lignin.
 17. The cellulose-rich composition of claim 14, wherein said composition comprises about 3 wt % or less hemicellulose.
 18. The cellulose-rich composition of claim 17, wherein said composition comprises about 2 wt % or less hemicellulose.
 19. The cellulose-rich composition of claim 14, wherein said composition comprises from about 1 wt % to about 2 wt % uronic acid groups.
 20. The cellulose-rich composition of claim 14, wherein said composition comprises about 2 wt % or less ash.
 21. The cellulose-rich composition of claim 14, wherein said composition is characterized by a ratio H/(C+H) of about 0.03 to about 0.04, wherein H is total hemicellulose and C is total cellulose.
 22. The cellulose-rich composition of claim 14, wherein said composition is produced by a process comprising: (a) providing a lignocellulosic biomass feedstock; (b) pre-extracting said feedstock in the presence of steam or hot water, thereby generating a first solids stream and a first liquid stream, wherein said first liquid stream contains hemicelluloses and lignin; (c) depositing at least some of said lignin, from said first liquid stream, onto a surface of said first solids stream to generate a lignin-coated intermediate material comprising cellulose-rich particles with a lignin coating; (d) optionally drying said lignin-coated intermediate material; (e) digesting said lignin-coated intermediate material in the presence of an acid, a solvent for lignin, and water, to generate a second solids stream and a second liquid stream, wherein during said digesting, the rate of delignification of surface lignin deposited from step (c) is lower than the rate of delignification of bulk lignin, thereby retaining at least a portion of said lignin coating; and (f) recovering said second solids stream as a lignin-coated cellulose-rich composition, wherein said lignin-coated cellulose-rich composition is at least partially hydrophobic.
 23. A cellulose-containing product comprising the cellulose-rich composition of claim
 14. 24. The cellulose-containing product of claim 23, wherein said cellulose-containing product is selected from the group consisting of a structural object, a foam, an aerogel, a polymer composite, a carbon composite, a film, a coating, a coating precursor, a current or voltage carrier, a filter, a membrane, a catalyst, a catalyst substrate, a coating additive, a paint additive, an adhesive additive, a cement additive, a paper coating, a thickening agent, a rheological modifier, an additive for a drilling fluid, and combinations or derivatives thereof. 